so there is a few things that can be the cause of your symptoms.
1) Be sure to make sure the device is either unlocked, or locked to your carrier.
2) Look into the national blacklist. If you got the device second hand, there is a chance it was reported stolen.
3) Software corruption. Run a factory reset on the device.
4) Disconnect. Make sure there is enough pressure on the metal traces lining the housings, to the board.
5) Coaxial breaks. Look for coaxial cables, test continuity, and replace them as needed. (make sure they arent loose first)
6) Antenna failure. Check for board antennas. sometimes a reflow of these parts will fix the issue.
7) Network IC failure. Board level failure is pretty common. you will need experience with SMT to replace the Components in the networking Circuit. most people will scrap at this level of failure.